The instant invention belongs to the field of printing, especially flexographic and gravure printing, and it is related to printing inks for this technical use.
Printing inks for gravure and flexographic printing have already been known for a rather long time. These inks comprise the following principal components:
at least one pigment, preferably an organic pigment; PA1 at least one binder, preferably naturally occuring or synthetic resins or rosins; and PA1 a solvent system. PA1 good dissolving power of the solid ink components, in particular the binder; PA1 an evaporation rate sufficiently high and, at the same time, a boiling point or boiling range sufficiently low to ascertain the necessary drying rate; PA1 chemical properties which are compatible with the intended use of the ink such as freedom of corrosive actions and low toxicity, PA1 chemical stability and compatibility with the other components of the printing ink.
Further optional components are fillers, optical brighteners, plasticizers, drying promoters, viscosity regulators, extenders, surface active agents, antifouling agents, etc.
Normally, these printing inks are rather of low viscosity, and the solvent system is relatively highly volatile. The drying of these printing inks in the printing process is based on the rapid evaporation of the solvent. After solvent evaporation, the binder (i.e. the resins) fix the finely divided pigments on the printed substrate. A good solubility of the binder in the solvent is very important, and any interaction between the ink solids, especially the binder, and the solvent molecules must remain as low as possible in order not to interfere with the drying process.
The desired rapid evaporation of the solvent generally excludes the use of water as a solvent. However, for special purposes, an organic solvent may be used which contains small amounts of water. Thus, "solvent", "solvent phase" or "solvent system" as used herein means at least one liquid organic compound, free of water under the conditions of normal use but which has not been specially dried and which may thus contain minor, random amounts of water, typically not more than about 2% by weight.
Flexographic and gravure printing inks are employed in the printing of various substrates such as paper and cardboard, particularly for packaging purposes; coated papers such as for prospectuses and illustrated newspapers and magazines; high quality printing paper such as for art reproductions; plastic films and sheets, especially for flexible packaging; laminates of transparent plastic films, for example made of polyester, polypropylene, hydrated cellulose ("Cellophane"), polyethylene, polyvinyl chloride, etc; aluminum foil and paper or plastic laminated aluminum films.
Normal and well known printing inks of the kind which are treated herein, are composed of about 10 to 60% by weight, based on the total weight of the ink, of total solids (i.e. all components, the solvent system expected), and 90 to 40% of the solvent or solvents. A preferred range is 20 to 50% by weight of total solids, and most inks contain 25 to 30% of solids, the remainder being essentially the solvent phase. As a solvent phase, any organic compound or compound mixture can be used which shows the necessary properties as a liquid vehicle for the printing ink, namely especially
Solvents useful for gravure and flexographic printing inks which are considered here, can be selected from a great number of organic liquids well known to the man skilled in the art. Examples for such liquids having in general a low boiling point, are the following: aliphatic hydrocarbons such as hexane, heptane, octane, cyclohexane, cycloheptane, and the different "boiling range" gasolines ("50/70", "60/80", "60/90" etc) where the figures denote the beginning and the end of boiling in .degree.C.; alcohols such as methanol, ethanol, n-propanol, i-propanol; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone; aromatic hydrocarbons such as toluene and xylene; aliphatic esters such as ethyl acetate, propyl acetate, isoproyl acetate; etc., and halogenated hydrocarbons, and mixtures of these liquids. The man skilled in the art is familiar with the choice of the particular printing ink solvent system.
Until now, flexographic and gravure printing inks have been manufactured according to a special method. First of all, the pigment which is supplied in powder form but which always tends to aggregate, is "formed" and prepared for the use in the ink. To do this, about equal parts of such a pigment and of a binder resin are ground together in a ball mill with the addition of some plasticizer and liquid solvent, until the aggregates have been broken up and the resin has formed a thin coating on every one of the pigment particles. Finally a liquid mass is obtained which is then further diluted with more solvent, under vigorous stirring, in special kettles until the final concentration is obtained. Before or during this further dilution, the other ink additives can be added to the liquid mixture.
The printing ink thus obtained is then transferred from the kettle into drums or other suitable containers and shipped to the printing houses where the ink will normally be stocked some time before use.
There are some problems associated with this process. First of all, the ball milling of pigments and resin is a time and energy consuming operation.
Secondly, the shipment of rather great quantities of liquid ink where about at least 75% by weight are highly inflammable solvents, is hazardous and very dangerous. Furthermore, relatively large containers must be used for shipping and the stockage of the liquid ink. Then, a great amount of solvents is to be shipped whereas normally, solvents are available everywhere and in particular at the location of the printing facilities. Finally, the stocked ink must be stirred before use since it may be possible that a portion of the solids, primarily the pigments, has settled and must be redispersed.
Dry, solid, powdery printing inks for gravure or flexographic printing which are to be reconstituted just before use by the addition of the solvents, have not been reported until now and are unknown to the printer. As will be described later on in more detail, this fact is not surprising since the man skilled in the art knew that the only technique of preparation of the ink was a wet one. It could not be imagined how pigments can be "formed" and incorporated into the printing ink formulation other than by a wet grinding step. Furthermore, the man skilled in the art knew that the sequence of addition of the components could not be selected at will but that a certain, predetermined sequence had to be imperatively observed.
For all these reasons, powdery flexographic and gravure printing inks are unknown until now.
Bray (U.S. Pat. No. 3,287,290) discloses a spray-dried paint composition. Of course, bearing in mind what has been said above, ink powders are not disclosed nor suggested.
Ferrill (U.S. Pat. No. 4,391,648) describes a pigment composition comprising 25 to 95% pigment and 5 to 75% resin. Inks are not disclosed nor suggested. Ferrill's composition can be used as one of the starting products (pre-dispersed pigment) in this invention.
Strong (U.S. Pat. No. 3,925,219) discloses a pressure-sensitive developing powder which has nothing to do with this invention and cannot be used for printing inks. The same applies to the disclosure of U.S. Pat. No. 3,965,022 (Strong et al.).